Henry Petroski’s “The Road Taken: The History and Future of America’s Infrastructure” is really two books in one. It is, first, a history of infrastructure from the Appian Way to the present. It also promises to be a guide for the present, helping us “better understand what is involved in making key choices that we are faced with today.” The book intermittently succeeds in both guises, though it takes some detours along the way.

Petroski, a professor of both engineering and history at Duke and the author of such books as “The Pencil” and “The Evolution of Useful Things,” brings an eye for the little things: what kinds of guardrails are best, how roads can be made safer through better signage, which paving materials last longest. One of his key lessons is that small thinking can be a virtue, because the history of infrastructure is a series of experimental and incremental improvements.

Local governments tried endless variations of asphalt and concrete before developing paving surfaces that didn’t produce excess dust or deteriorate quickly under rain and snow. They gradually built longer bridges, learning from earlier designs that worked, and that didn’t. They tried out different paint colors for lane markings, finding the ones that drivers could see best.

This little-things perspective is needed at a time when America’s infrastructure agenda is simultaneously characterized by grandiose ambitions and limited budgets. Money is tight, and infrastructure needs are going unaddressed. At the same time, despite funding limitations, politicians have a tendency to fall in love with novel, pathbreaking, expensive projects that frequently go astray, resulting in arguments against spending more on infrastructure.

Petroski devotes one chapter of his book to the new eastern span of the San ­Francisco-Oakland Bay Bridge, which opened in 2013, nine years late and $5 billion over budget. “With uniqueness also come uncertainties — of complications during design and construction and of cost,” he writes. Replacing an old bridge with seismic problems could have been done fairly easily and cheaply by building a simple viaduct. But politicians wanted a “signature span,” and for a variety of aesthetic reasons they chose to build a single-tower, self-anchored suspension bridge — a relatively rare design. The proposed bridge would be the longest of its kind in the world.

But self-anchored suspension bridges lack the massive anchorages at each end that are typical for suspension bridges. Instead, the cables would be anchored to the deck itself. Because of the desire to add a cantilevered bike lane, the bridge would also have to be wider on one side than the other.

This combination of specifications led to a variety of unforeseen complications. The addition of the asymmetrical bike lane required a counterweight, which would increase the load on the bridge cables, which would pull on the deck, which therefore had to be built stiffer to resist the stronger pull. But the stiffening would make the deck heavier, further increasing the load on the cables, requiring further stiffening, and so forth.

These shifting specifications added greatly to time and cost, obliterating the justification that had led politicians to choose to build a new bridge in the first place: that it would cost about the same amount as retrofitting the old span to be safer in earthquakes. And in the end, the single tower wasn’t built quite upright, and the technique used to straighten it after construction weakened the steel rods inside it, calling into question how seismically sound it was anyway.

Politicians aren’t drawn to megaprojects just because they believe the initial rosy cost projections and therefore underestimate the risk of complications. They also see an opportunity to build their legacy: It’s more fun to say “I built that bridge” than “I retrofitted that bridge.”

In New York, we have just celebrated the opening of the world’s most expensive train station, a $4 billion replacement for an existing subway terminal at the World Trade Center. Gov. Andrew M. Cuomo has also revived a decades-old plan for a miles-long tunnel under Long Island Sound. Yet nobody, including the governor, has found a way to fund the Metropolitan Transportation Authority’s five-year capital plan, filled with more quotidian projects like signal upgrades, which do not lend themselves so well to ribbon-cutting photo opportunities.

None of this is to say infrastructure spending is always pointless or excessive. On the contrary, Petroski makes a strong case that we face an infrastructure deficit, at the same time that politicians have eroded voters’ trust by mismanaging the megaprojects. In particular, we have often neglected the small stuff, allowing roads to decay to the point where cheap ­patching won’t do and expensive rebuilding is necessary. Petroski describes road maintenance procedures in some detail, arguing that $1 of prevention for a mildly deteriorated road could save $4 or $5 a few years later.

As is Petroski’s style, the book contains many suggestions to make our infrastructure a little better and a little safer: Guardrails should be designed with their ends buried in the ground so that vehicles can’t crash into them, for example; missing safety signs should be replaced promptly; the government should develop lane markings that are easily visible in snow yet can’t be ripped up by snowplow blades.

The discussions of roads and bridges largely deliver on the promise of providing a road map for current policy debates, from big national issues like federal highway funding to ones you’ve probably never thought of, like the lane markings issue. Unfortunately, other parts of the book wander without a clear destination. Through the book, Petroski uses his personal recollections of infrastructure to illustrate current issues in civil engineering. But many of the recollections could have been bypassed.

“Generally speaking, trucks use our road only when a delivery is being made,” he writes about his own neighborhood. “Once, an open-sided truck brought and left a herd of goats hired to munch away on ivy that had taken over an undeveloped lot across the street. At the end of the day the truck returned to pick up the sated grazers.”

As far as I can tell, the goat story has no broader implication for the reader. Neither does Petroski’s musing about why his former home street was called an avenue when it was only two blocks long. A story about how the storm drain in front of his home used to get clogged with leaves is equally detached from any larger question.

There are other problems, too. Petroski makes a weak and impressionistic argument that the construction quality of residential structures has declined over time, reprising an argument he made in 2014 on the Op-Ed page of The New York Times. At the time, Paul Krugman wrote that he was unconvinced, and raised the issue of survivorship bias: Most buildings don’t last 100 years, and the ones that do tend to be those that were built well in the first place. So when you look at an old building today, you tend to be looking at a building that was unusually well built for its time. Petroski acknowledges Krugman’s argument but doesn’t really address it, falling back again on “anecdotal evidence from owners and the structures themselves” to contend that the quality of private construction has worsened.

For enthusiasts of infrastructure and voters who are concerned about “our crumbling roads and bridges” and what to do about them, Petroski provides valuable historical context to inform today’s policy debates. But they may find themselves skipping ahead through some of the personal stories to get to the meat of the book.